The embodiments of the present invention relate generally to methods and apparatus for subsea control systems. More particularly, the embodiments of the present invention relate to control systems for subsea chokes. More particularly, the embodiments of the present invention relate to control systems for improving the response time, controllability, uptime availability, and retrievability of the active components of subsea chokes.
In offshore oil and gas production, it is often common for more than one well to be produced through a single flowline. In a typical installation, the products from each individual well flow are combined into a common flowline, which then carries the products to the surface or combines those products with the products of other flowlines. The difficulty in managing a multiple well completion produced through a single flowline is that not all of the wells may be producing at the same pressure conditions or include the same flow constituents (liquids and gases).
For example, if one individual well is producing at a lower pressure than the pressure maintained in the flowline, fluid can backflow from the flowline into that well. Not only is the loss of production fluids undesirable, but the pressure changes and reverse flow conditions within that well may damage the well and/or reservoir. Similarly, if one well is producing at a pressure above the flowline pressure, that well may produce at an undesirable flow rate and pressure, again with the potential to damage other wells and/or the reservoir. Thus, the management of flow rates and pressures is of critical importance in maximizing the production of hydrocarbons from the reservoir.
Prior art subsea production systems, including a choke 15, are shown in FIGS. 1-3. Referring initially to FIG. 1, control signals and a hydraulic fluid supply are transmitted along an umbilical 30 from a topside control system 20 to a subsea control module 40, which supplies hydraulic fluid to actuators in the subsea trees, manifolds, valves, and other functions along lines 60. As control valves within the control module 40 receive signals to open or close the choke, the control valves actuate to control the flow of hydraulic fluid to the choke actuator 17 through either hydraulic line 16, for opening, or hydraulic line 18, for closing. The common choke actuator 17 is a hydraulic stepping actuator, which, depending on the style of actuator and choke being used, may take 100 to 200 steps to close, although systems requiring a smaller, or larger, number of steps are possible. Each step involves the actuator 17 receiving a pulse of hydraulic pressure, which moves the actuator, and then a release of that pressure, which allows a spring to return the actuator to its initial position. In typical systems, where the SCM is located proximate (e.g., within about 30-feet) to the choke/actuator, about one second is required for the pressure pulse to travel from the control valve in module 40 to the actuator 17 and two seconds are required for the spring to return the actuator to its initial position. Thus, with a total of three seconds per step and a total of up to 200 or more steps required to fully actuate the choke, the time required to fully close or open the choke is considerable. The risk of equipment failure is also increased due to the components being actuated hundreds, thousands, or even millions, of times.
Another typical prior art subsea production system, including a choke 15, is shown in FIG. 2. Control signals and a hydraulic fluid supply are transmitted along an umbilical 32 from a topside control system 20 directly to a subsea choke 15, bypassing subsea control module 40 on an electro hydraulic control system. Operation of a direct hydraulic control system would also be as described above, since no subsea control module is required, and a direct electric (control) system would operate similarly, minus any hydraulic control lines. The choke 15 is opened and also closed via hydraulic signals transmitted through dedicated umbilical lines. Hydraulic signals from the surface control the flow of hydraulic fluid to the choke actuator 17 through either hydraulic line 16, for opening, or hydraulic line 18, for closing. The common choke actuator 17 is a hydraulic stepping actuator which, depending on the style of actuator and choke being used, may take 130-180 steps to close. Each step involves the actuator 17 receiving a pulse of hydraulic pressure, which moves the actuator, and then a release of that pressure, which allows a spring to return the actuator to its initial position. In typical systems, the time required for the pressure pulse to travel from the surface to the actuator 17 is directly related to the offset distance (umbilical length from surface to choke), water depth and actuating pressure, which can be minutes per step for long offsets. Also, an additional amount of time is required for the spring to return the actuator to its initial position. The time to actuate each step can run into minutes, thus, with a total of up to 180 steps required to fully actuate the choke, the time required to fully close or open the choke is considerable.
A third typical prior art subsea production system, including a choke 15, is shown in FIG. 3. Electrical power and a hydraulic fluid supply are transmitted along an umbilical 34 from a topside control system 20 directly to a subsea choke actuator system 22, bypassing subsea control module 40 on an electro hydraulic control system. Operation of a direct hydraulic control system would also be as described above, since no subsea control module is required, and a direct electric (control) system would operate similarly, minus any hydraulic control lines. A hydraulic fluid supply is stored local to the choke 15, such as in accumulator 28. The choke 15 is opened and also closed via electrical signals transmitted through dedicated umbilical conductors 26 and 27 to actuate the open and close functions. The electrical signals are received by a directional control valve 38 that regulates hydraulic flow to the open and close functions of choke actuator 17. For this instance, hydraulic fluid is supplied to the local choke accumulators 28, which are refilled by the hydraulic supply along umbilical 32. The common choke actuator 17 is a hydraulic stepping actuator which, depending on the style of actuator and choke being used, may take 100 to 200 steps to close. Each step involves the actuator 17 receiving an electrical power pulse, followed by a pulse of hydraulic pressure, which moves the actuator, and then a release of the electrical power that releases the hydraulic pressure, which allows a spring to return the actuator to its initial position. In typical systems, roughly one second is required for the electrical power pulse to travel from the surface to the choke, and then for the pressure pulse to travel from the local choke accumulator to the actuator 17 and roughly two seconds are required for the spring to return the actuator to its initial position. Thus, with a total of three to four seconds per step and a total of up to 180 steps required to fully actuate the choke, the time required to fully close or open the choke is considerable. The power requirements for this type of system are considerable, while the umbilical must have electrical conductors 26 and 28 (one for open, one for close) for each choke.
Thus, there remains a need in the art for methods and apparatus for increasing the responsiveness and speed of choke control systems, especially subsea systems. Therefore, the embodiments of the present invention are directed to methods and apparatus for controlling choke actuation that seek to overcome the limitations of the prior art.